The present invention relates to a method and apparatus for depositing standard or known size particles on a semiconductor wafer or other surface without mechanical movement of the wafer during the deposition process while precisely controlling the depositing of particles.
In the semiconductor industry, it is often necessary to determine the number of particles on a wafer by use of a wafer inspection tool. Such tools generally operate on the principle of light scattering, which measures the particle size on the wafer by the amount of light scattered by each particle as a laser beam is swept across the wafer surface. To standardize such an inspection tool it is necessary to deposit particles of a known size on a wafer and place the wafer in the inspection tool to determine the response from that reference wafer particles, for comparison with known factory settings of the inspection tool. It is also important to know the number of particles on the wafer, and to have a reasonably uniform distribution of particles on the wafer surface. This will essentially be a calibration process to ensure that the results obtained from the wafer inspection tool are accurate or in some way can be correlated to standard readings.
In addition, the original calibration of the inspection tool in the factory is also based on the use of known size particles on a wafer. The most commonly used calibration particles are polystyrene latex (PSL) spheres, usually in the size range between 0.1 .mu.m and 2 .mu.m. Also, there is a need to deposit uniform size particles of silicon, silicon dioxide, silicon nitride and similar materials on wafers, to determine the response of the inspection tool to particles of a different material. See for example the article by S. A. Chae, H. S. Lee and B. Y. H. Liu "Size Response Characteristics Of A Wafer Surface Scanner for Non-Ideal, Real-World Particles" Journal of the IES 35(6):45-52, December 1992.
A commonly used method of depositing such standard calibration particles on a wafer is to make a suspension of the particles, such as PSL, in water, and then atomize the suspension to form a spray. The spray is allowed to dry leaving uniform sized PSL particles suspended in air to form an aerosol. The aerosol is then introduced into a chamber that holds the wafer to permit these particles to deposit on the wafer surface. The mechanism that causes particles to deposit on a wafer from an aerosol are well known. Theoretical and experimental studies have shown that the principle deposition mechanisms are gravitational settling, and diffusion. If the particles are electrically charged, electrostatic attraction also plays a role in particle deposition. See the article by B. Y. H. Liu and K. H. Ahn, entitled "Particle Deposition on Semiconductor Wafers," Aerosol Sci. Technol. 6:215-224 1987 and the article by D. Y. H. Pui, et al. "Experimental Study of Particle Deposition on Semiconductor Wafers," Aerosol Science and Technology 12:795-804, 1990.
To carry out the deposition in a chamber containing a wafer by the simple introduction of an aerosol into the chamber has a number of disadvantages. First, when the wafer is placed in the chamber, the uncontrolled contaminate particles already in the chamber can deposit on the wafer to cause unwanted contamination of the wafer. Second, when the aerosol is first introduced into the chamber, the particle concentration in the chamber will build up slowly as the aerosol is mixed with the relatively clean chamber air. The rate of particle deposition, therefore, will vary as a function of time due to the varying particle concentration. The varying particle concentration makes it difficult to control the number of particles deposited on the wafer. Similarly, when the aerosol flow into the chamber is stopped, the particle concentration in the chamber will slow decay, again causing variations in particle deposition rate with time and making it difficult to control or know the number of particles deposited.
U.S. Pat. No. 5,194,297 entitled "SYSTEM AND METHOD FOR ACCURATELY DEPOSITING PARTICLES ON A SURFACE" by Bradley H. Scheer, et al. discloses a method of attempting to overcome such difficulty by providing a clean sheath flow area in the chamber. That is, there is a region where there is clean air providing a sheath over the wafer, and a separate aerosol flow area is provided in the same deposition chamber. The wafer is first introduced into the clean sheath flow area prior to deposition, and is protected from deposition of particles when in the clean sheath flow area. The wafer is then moved mechanically into the aerosol flow area for deposition, and then back to the clean sheath flow area following deposition, to complete the cycle. Mechanical movement of the wafer in a deposition chamber is complicated, and can cause unwanted contaminate particles to be generated by the mechanical movement of a conveyor belt, a robot arm or other gear mechanism needed to move the wafer between the two regions within the chamber.
The present invention seeks to overcome the problems inherent in accurately depositing known size particles onto a wafer, without introducing unwanted contaminates, in a simple, easily controlled method.